Electrochromic materials are substances which change color under electrical stimulation. Electrochromic material is one color in its reduced state and another color in its oxidized state. Most electrochromic materials change color through a redox reaction that is driven by electric current flowing across an electrolyte --electrochrome interface.
Representative reaction mechanisms for redox electrochromic materials include those for Prussian Blue (ferric ferrocyanide), V. D. Neff, J. Electrochem. Soc., 125, 886 (1978), lutetium diphthalocyanine [Margie M. Nicholson, Ind. Eng. Chem. Prod. Res. Dev. 21, 261 (1982)]; viologens [B. Reichman, Fu-Ren F. Fan, and A.J. Bard, J. Electrochem. Soc., 127, 333 (1980)], tungsten oxides [B. Reichman and A.J. Bard, J. Electrochem. Soc., 126, 583 (1979)], and others.
Several authors have mentioned possible display applications for electronically addressable systems involving electrochromic material on a conductive substrate [Margie M. Nicholson, Ind. Eng. Chem. Prod. Res. Dev. 21, 261 (1982)]; e.g., electronically addressable electrochromic displays using tungsten oxides have been suggested for timepieces, according to some researchers (Id. at 265). The major problem with fabrication of these displays, however, was the complexity of matrix addressing systems necessary to achieve the flexibility required for a plurality of displays.
The photoelectrochromic effect of semiconductor--metallic ion solution interfaces was observed by researchers [T. Inoue, A. Fujishima and K. Honda, Chem. Lett., 11, 1197 (1978)]. In these metallic ion systems, the metals are deposited on the semiconductor electrode from the solution by a redox mechanism under a proper electrical bias by irradiation at an energy level above the bandgap of the semiconductor. The radiation creates charge carriers in the semiconductor that drive the redox reaction at the interface. When the bias on the electrode is reversed, the redox reaction is reversed and the metal returns to solution. Thus, the semiconductor electrode undergoes a photoelectrochromic effect because the precipitated metal covering the electrode appears as a change in the color of the electrode.
A similar photoelectrochromic effect was observed for a viologen solution semiconductor interface (B. Reichman, Fu-Ren F. Fan, and A. J. Bard, J. Electrochem. Soc., 127, 333 (1980). Using a p-GaAs electrode in a solution of heptyl viologen and bromide ions, the deposition of heptyl viologen bromide on the electrode through a redox reaction driven by photo generated charge carriers caused a color change to appear on the electrode. The author suggested that this photoelectrochromic system might have display applications if a laser raster system could be developed to control and drive the color changes. It was also suggested that electrochromic viologen polymer films formed on a p-type semiconductor electrode could be used in photoelectrochromic displays in a like manner to the viologen solution system [H. D. Abruna and A. J. Bard, J. Am. Chem. Soc., 103, 6398 (1981)]. However, these polymer films degraded quickly and were limited to use on p-type semiconductors. Further, a suitable matrix addressing system has not, so far as is known, been developed.